As early as Ford U.S. Pat. No. 2,691,134, it has been known to sense a liquid hydrocarbon, such as gasoline, by means of a sensor of a type comprising a rubber strip having carbon black particles dispersed therethrough so as to be capable of switching from a state of relatively low electrical impedance to a state of relatively high electrical impedance in the presence of a liquid hydrocarbon. The rubber strip absorbs the liquid hydrocarbon and swells in the presence of the absorbed hydrocarbon.
A sensor of the type noted above is available commercially from One Plus Corp. of Northbrook, Ill., under its LEAK EDGE trademark. The commercially available sensor has an inner, electrically conductive layer and an outer, electrically insulative layer. Each of the inner and outer layers is made from a silicone rubber, which is capable of absorbing any of a wide range of liquid hydrocarbons and swelling in the presence of the absorbed hydrocarbon. The inner layer but not the outer layer has carbon black particles dispersed therethrough so as to be capable of switching from a state of relatively low electrical impedance to a state of relatively high electrical impedance when swollen in the presence of a liquid hydrocarbon. Being electrically insulative, the outer layer prevents short circuiting of the sensor in water.
Since the sensor described in the preceding paragraph does not float on water or other liquids of low viscosity, its utility is limited in sewers, marine bilges, and other locations where water tends to flow so as to wash a liquid hydrocarbon from such a sensor or where liquids tend to accumulate at varying depths. This invention has resulted from efforts to enhance the utility of a sensor of the type noted above.
Sensors of related interest are disclosed in Dolan et al. U.S. Pat. No. 3,045,198, Kishikawa et al. U.S. Pat. No. 3,970,863, Dolan U.S. Pat. No. 4,129,030, Dolan U.S. Pat. No. 4,224,595, Dolan U.S. Pat. No. 4,237,721, Donaghey U.S. Pat. No. 4,631,952, Hauptly U.S. Pat. No. 4,885,706, Akiba U.S. Pat. No. 5,109,202, and Neuburger U.S. Pat. No. 5,256,574.